The present invention relates to a new charge generation material for organophotoconductors, in particular, to an improved bisazo compound and a squarylium compound as the charge generation material for novel, high performance organophotoconductors in copy machines or laser printers.
Copy machines play important roles in office automation and provide fast and convenient copy services. The copy process utilizes xerography as discovered by C. Carlson in 1938. Based on this technology of producing high quality permanent images, the xerographic process comprises six steps, namely, (1) photoconductor charging, (2) image exposure, (3) image development, (4) image transferring, (5) fixing of image, and (6) cleaning of photoconductor. The photoconductor, similar to the human heart, has a major role in the copy process.
Normally, photoconductors can be classified as inorganic or organic. Conventional photoconductors used in copy machines, for example, selenium, cadmium sulfide, zinc oxide, etc., are inorganic. However, organophotoconductors are gradually being adopted, becoming commercially available and widely applied in laser printers and copy machines. The advantages of using organophotoconductors are (1) no toxic pollution from inorganic metals, (2) low cost of manufacturing, (3) easy manufacture of different shapes, e.g. drum, belt and sheet, and (4) clear image production. However, the sensitivity, the stability towards light and heat and the durability of organophotoconductors are still lower than those of inorganic photoconductors. Thus, organophotoconductors are now mainly used in copy machines of low or middle copying speed.
The so-called organophotoconductor is an insulator in darkness. After exposure in light, it will absorb photons to separate charges and form electron-hole pairs, wherein electrons and holes move towards different directions in an externally applied magnetic field in becoming a conductor. In other words, the organophotoconductor will produce charges while lighting and maintain the charges on a surface in darkness; but it cannot be quickly photoneutralized and should possess charge transport structures. With development, the structures of organophotoconductors are classified as monolayer type or functionally separated laminated type. The latter, the most preferred type, contains a conductive substrate, a charge generation layer and a charge transport layer. When the charge generation layer is exposed to light, it is capable of producing electron-hole pairs. The charge transport layer is responsible for transporting electrons or holes onto the surface of organophotoconductors. Moreover, the charge generation layer is composed of very small particles of charge generation materials evenly dispersed in a polymer binder. Generally, the charge generation materials are organic pigments, such as squarylium pigment, phthalocyanine pigment, bisazo pigment, etc. The effect of the charge generation materials is absorbance of injected light and production of electrons. Thus, the organic pigments should possess an excellent degree of absorbance towards the selected light. Fluorescent light is normally used in copy machines and emits visible lightwaves of 450 nm to 650 nm. That is, the charge generation materials of organophotoconductors used in copy machine possess excellent sensitivity towards the above visible lightwaves. In particular, due to their advantages in good sensitivity and selectivity towards visible light, bisazo pigments are widely used in organophotoconductors for the range of visible light. A lot of U.S. patents relate to bisazo pigments, but commercially available bisazo pigments for photoconductors in these patents are rare. The major reason is that the pigments do not result in satisfactory sensitivity, stability towards light and heat and durability. In particular, such pigments lack absorbent capabilities towards near-infrared light with a wavelength of 700 nm to 850 nm and cannot be applied in laser printers.
The organic pigments, such as squarylium pigment, can absorb near-infrared light. U.S. Pat. No. 3,824,099 discloses a squarylium pigment having an excellent sensitivity of absorbance but high dark decay and extremely low charge acceptances are the results in synthesis process. Although the squarylium pigment possesses sensitivity towards near-infrared light, it cannot be practically applied in organophotoconductors.
Almost more than one hundred patents relate to charge generation materials for organophotoconductors, but only a few relate to mixtures composed of two or more organic pigments as charge generation materials. More particularly, there is no disclosure about mixtures of squarylium pigment, and bisazo pigment as charge generation materials as in the present invention. The advantages and drawbacks of U.S. Pat. No. 4,353,971 and U.S. Pat. No. 4,983,483 and Japan Patent 02,154,272, which are relevant to this application, Will be discussed below. The characteristics of U.S. Pat. No. 4,353,971 are utilization of a mixture of different squarylium pigments and diamine blue dyes as the charge generation material and use of a organic compound, such as hydrazones or 2-pyrazolines, coated thereon as the charge transport layer to form organophotoconductors. The range of wavelength absorbance for the charge generation materials disclosed in the patent is 400 nm to 1150 nm of visible light and near-infrared light, but the sensitivity thereof is only 132 uJ/cm.sup.2 which cannot satisfy practical use requirements. The ideal value of sensitivity should be within 1 uJ/cm.sup.2.
The key point of US Patent No. 4,983,483 is utilization of a mixture of squarylium pigments and copper phthalocyanine pigments which is dry milled in a ball mill to transfer the crystal shape and then wet milled in a sand mill for 20 hours to form charge generation materials. The sensitivity of organophotoconductors obtained at near-infrared light is 0.5 uJ/cm.sup.2, but that in the range of visible light is still not high enough for use in copy machines. The characteristic of Japan Patent No. 02,154,272 is use of a mixture of two kinds of bisazo pigments as charge generation materials. The organophotoconductors produced have a wide range of absorbance, from 450 nm to 850 nm. The sensitivity of the patent will reach 1.5 Lux.sec to 1.8 Lux.sec. But the synthesis for each bisazo pigment in the mixtures really wastes time. The difficulty in purifying is noted and, especially the manufacture of organophotoconductors in large scale research or for commercial purpose is greatly challenged.
In order to improve the drawbacks of the above patents, the present invention provides a new bisazo pigment which can absorb the wavelengths of visible light and furthermore, a new charge generation material for organophotoconductors by mixing the bisazo pigment and a squarylium pigment which can absorb the wavelength region of near-infrared light. The new charge generation materials in the invention will absorb both visible light and near-infrared light, have excellent photosensitivity, charge acceptance, stable photoconductivity and durability, and then can be used in commercially available organophotoconductors for copy machines or laser printers.